1. Field of the Invention
This invention relates to an electron beam apparatus and an image forming apparatus such as a display apparatus realized by using the same. More particularly, the present invention relates to an electron beam device and an image forming apparatus comprising an envelope and spacers for supporting and reinforcing the envelope from inside to make it withstand the atmospheric pressure.
2. Related Background Art
There have been known two types of electron-emitting devices; the thermionic cathode type and the cold cathode type. Of these, the cold cathode type refers to devices including surface conduction electron-emitting devices, field emission type (hereinafter referred to as the FE type) devices and metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices.
Examples of surface conduction electron-emitting devices include one proposed by M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965) as well as those that will be described hereinafter.
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of SnO2 thin film for a device of this type, the use of Au thin film is proposed in G. Dimmer: “Thin Solid Films”, 9, 317 (1972) whereas the use of In2O3/SnO2 and that of carbon thin film are discussed respectively in M. Hartwell and C. G. Fonstad: “IEEE Trans. ED Conf.”, 519 (1975) and H. Araki et al.: “Vacuum”, Vol. 26, No. 1, p. 22 (1983).
FIG. 36 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In FIG. 36, reference numeral 3001 denotes a substrate. Reference numeral 3004 denotes an electroconductive thin film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which eventually makes an electron-emitting region 3005 when it is subjected to an electrically energizing process referred to as “energization forming” as described hereinafter. In FIG. 36, the thin horizontal area of the metal oxide film separating a pair of device electrodes has a length L of 0.5 to 1 mm and a width W of 0.1 mm. Note that, while the electron-emitting region 3005 has a rectangular form and is located at the middle of the electroconductive thin film 3004, there is no way to accurately know its location and contour.
For surface conduction electron-emitting devices including those proposed by M. Hartwell et al., the electroconductive film 3004 is normally subjected to an electrically energizing preliminary process, which is referred to as “energization forming”, to produce an electron emitting region 3005. In the energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a rate of 1 V/min. is applied to given opposite ends of the electroconductive film 3004 to partly destroy, deform or transform the thin film and produce an electron-emitting region 3005 which is electrically highly resistive. Thus, the electron-emitting region 3005 is part of the electroconductive film 3004 that typically contains fissures therein so that electrons may be emitted from those fissures. Note that, once subjected to an energization forming process, a surface conduction electron-emitting device comes to emit electrons from its electron emitting region 3005 whenever an appropriate voltage is applied to the electroconductive film 3004 to make an electric current run through the device.
Examples of FE type device include those proposed by W. P. Dyke & W. W. Dolan, “Field emission”, Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, “PHYSICAL Properties of thin-film field emission cathodes with molybdenum cones”, J. Appl. Phys., 47, 5248 (1976).
FIG. 37 of the accompanying drawings illustrates in cross section an FE type device according to the above C. A. Spindt paper. Referring to FIG. 37, the device comprises a substrate 3010, an emitter wiring 3011, an emitter cone 3012, an insulation layer 3013 and a gate electrode 3014. When an appropriate voltage is applied between the emitter cone 3012 and the gate electrode 3014 of the device, the phenomenon of field emission appears at the top of the emitter cone 3012.
Apart from the multilayer structure of FIG. 37, an FE type device may also be realized by arranging an emitter and a gate electrode on a substrate substantially in parallel with the substrate.
MIM devices are disclosed in papers include C. A. Mead, “Operation of tunnel-emission Devices”, J. Appl. Phys., 32, 646 (1961). FIG. 38 illustrates a typical MIM device in cross section. Referring to FIG. 38, the device comprises a substrate 3020, a lower electrode 3021, a thin insulation layer 3022 as thin as 100 angstroms and an upper electrode having a thickness between 80 and 300 angstroms. Electrons are emitted from the surface of the upper electrode 3023 when an appropriate voltage is applied between the upper electrode 3023 and the lower electrode 3023 of the MIM device.
Cold cathode devices as described above do not require any heating arrangement because, unlike thermionic cathode devices, they can emit electrons at low temperatures. Hence, the cold cathode device is structurally by far simpler than the thermionic cathode device and can be made very small. If a large number of cold cathode devices are densely arranged on a substrate, the substrate is free from problems such as melting by heat. Additionally, while the thermionic cathode device takes a rather long response time because it operates only when heated by a heater, the cold cathode device starts operating very quickly.
Therefore, studies have been and are currently being conducted on cold cathode devices.
For example, since a surface conduction electron-emitting device has a particularly simple structure and can be manufactured in a simple manner, a large number of such devices can advantageously be arranged on a large area without difficulty. As a matter of fact, a number of studies have been made to fully exploit this advantage of surface conduction electron-emitting devices. Studies that have been made to arrange a large number of devices and drive them effectively include the one described in Japanese Patent Application Laid-Open No. 64-31332 filed by the applicant of the present patent application.
Electron beam apparatuses using surface conduction electron-emitting devices that are currently being studied include charged electron beam sources and image forming apparatuses such as image displays and image recorders.
U.S. Pat. No. 5,066,883, Japanese Patent Application Laid-Open Nos. 2-257551 and 4-28137 also filed by the applicant of the present patent application disclose image display apparatuses realized by combining surface conduction electron-emitting devices and a fluorescent panel that emits light as it is irradiated with electron beams. An image display apparatus comprising surface conduction electron-emitting devices and a fluorescent panel can be highly advantageous relative to comparable conventional apparatuses such as liquid crystal image display apparatuses that have been popular in recent years because it is of a light emissive type which requires no backlight to make it glow and has a wide view angle.
On the other hand, U.S. Pat. No. 4,904,895 of the applicant of the present patent application discloses an image display apparatuses realized by arranging a large number of FE type devices. Other examples of image display apparatus comprising FE type devices include the one reported by R. Meyer R. Meyer: “Recent Development on Microtips Display at LETT”, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, p.p 6-9 (1991).
Japanese Patent Application Laid-Open No: 3-55738 also filed by the applicant of the present patent application describes an image display apparatus realized by arranging a large number of MIM type devices.
Image display apparatuses and other electron beam apparatuses described above normally comprise an envelope for maintaining the inside of the apparatus in a vacuum condition, an electron source arranged within the envelope, a target to be irradiated with electron beams emitted from the electron source and an accelerating electrode for accelerating electron beams heading for the target. In certain cases, such an apparatus additionally comprises one or more than one spacers arranged within the envelope for supporting the envelope from the inside in order to counter the atmospheric pressure applied to the envelope.
In particularly, in view of the current trend of the ever increasing demand for image display apparatuses and other image forming apparatuses that are very flat and have a large display screen, spacers within the envelope of display apparatus seems to be an indispensable component of such an apparatus.
However, spacers arranged within an electron beam apparatus can give rise to a problem of displacing the landing positions of electron beams from the respective designed positions on the plane where the target is arranged.
If the electron beam apparatus is a display apparatus of any of the above described types, the above problem may be expressed in terms of displaced landing positions and deformed contours of glowing spots on the surface of the fluorescent panel that are different from the designed ones.
When a color image forming panel that carries thereon fluorescent members of red, green and blue is used in such an apparatus, displaced landing positions of electron beams can result in a reduced brightness and color change. These problems are particularly observable around the spacers between the electron beam source and the image forming panel and in the peripheral areas of the image forming panel.